Synthesis, Structure, and Reactivity of Iridium NHC Pincer Complexes

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Synthesis, Structure, and Reactivity of Iridium NHC Pincer Complexes Katherine M. Schultz,† Karen I. Goldberg,† Dmitry G. Gusev,‡ and D. Michael Heinekey*,† † ‡

Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States Department of Chemistry, Wilfrid Laurier University, Waterloo, Ontario N2L 3C5, Canada

bS Supporting Information ABSTRACT: Iridium pincer complexes of the form (C∧C∧CMes)IrCl2 (1), (C∧C∧CMes)IrH4 (2), and (C∧C∧CMes)Ir(H)2L (L = PMe3 (3), pyridine (4)) have been prepared (C∧C∧C = [κ3-1,3-(CH2NHCMes)2C6H3]; NHCMes = N-mesitylimidazol-2-ylidene). Complexes 1-4 are the first examples of iridium complexes with CCC pincer ligands containing methylene bridges. Complex 2 activates C-H bonds of arenes at room temperature, as demonstrated by isotope exchange reactions. Under analogous conditions, no reaction was observed with alkanes.

’ INTRODUCTION Ligands enforcing a tridentate meridional or pseudomeridional binding geometry are collectively known as pincer ligands. Pincer complexes of transition metals have been studied intensively in recent years, and a wide range of complexes have been reported.1 The steric and electronic tunability of this structural motif and the variety of demonstrated reactivity make these complexes a promising choice in the development of selective catalysts for alkane functionalization.2 The majority of pincer ligands employ phosphine or amine moieties as neutral twoelectron donor atoms; many transition metal complexes of this type have demonstrated catalytic activity for a range of transformations.3 Of particular interest are the complexes (PCP)IrH2 (PCP = [κ3-1,3-(CH2PtBu2)2C6H3]) and (POCOP)IrH2 (POCOP = [κ3-1,3-(OPtBu2)2C6H3]), which catalyze CH activation and dehydrogenation reactions (Figure 1).4 (POCOP)IrH2 has also shown high catalytic activity in the dehydrogenation of amine boranes, which has implications in the field of hydrogen storage materials.5 The ability to easily dehydrogenate and functionalize alkanes would allow better use of low molecular weight hydrocarbons such as those found in natural gas reserves. The products of such transformations are desirable as starting materials to make higher value chemicals. We hoped to improve upon these catalysts in the areas of oxidative stability and catalytic activity by combining the pincer motif with N-heterocyclic carbene (NHC) ligands. While generally thermally stable, phosphine ligands may undergo r 2011 American Chemical Society

decomposition under oxidizing conditions.6 Substitution of P-donor atoms with NHC moieties is expected to give complexes with increased oxidative stability. Furthermore, intermediates resulting from C-H bond activation, thought to be in high oxidation states, may be better stabilized by NHC ligands than by less electron-donating phosphines.7 Until recently, there have been surprisingly few examples of CCC pincer complexes in general and CCC pincer complexes of cobalt-group metals in particular.8 One reason for the relative scarcity of the CCC motif is the synthetic difficulty presented by the need for deprotonation of the carbene precursors and activation of the aryl backbone. Formation of CCC complexes containing Zr or Pd has been aided by the availability of metal starting materials in low oxidation states with basic ligands. These starting materials can facilitate these steps simultaneously by both acting as a base and promoting oxidative addition of the aryl C-H bond.9 The first example of an iridium CCC pincer complex was synthesized via transmetalation from a Zr intermediate by Hollis and co-workers in 2008 (Figure 2).10 The resulting [(CCC)IrI2]2 dimer demonstrated catalytic activity in the intramolecular hydroamination of secondary amines. A more direct preparation of the same compound in lower yield was reported by Braunstein,

Received: October 29, 2010 Published: March 02, 2011 1429

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Figure 1. Coordination geometry of previously reported PCP and POCOP pincer complexes in comparison to CCC and C∧C∧C pincer complexes.

Figure 3. ORTEP of 1-Cl, showing 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond angles (deg) and lengths (Å): C(1)-Ir-C(12) 91.4, C(1)-Ir-C(9) 90.9, C(1)-IrCl(1) 89.8, C(1)-Ir-Cl(2) 93.6, C12-Ir-Cl(1) 89.3, C12-Ir-Cl(2) 90.1, C(9)-Ir-Cl(1) 90.8, C(9)-Ir-Cl(2) 89.4, Cl(1)-Ir-Cl(2) 176.4, C(12)-Ir-C(9) 177.6, Ir-C(1) 1.999, Ir-C(9) 2.030, Ir-C(12) 2.019, Ir-Cl(1) 2.379, Ir-Cl(2) 2.387.

Figure 2. (CCC)Ir pincer-type complexes.

who also described isolation of the first mononuclear (CCC)Ir pincer-type complexes.11 During the preparation of this article, the first (CCC)Ir pincer complexes to demonstrate catalytic C-H functionalization and dehydrogenation of alkanes were reported by Chianese et al.12 All of the Ir(CCC) complexes described above fall into the class of pincer ligands that lack a CH2 (or other group) spacer between Scheme 1. Synthesis of 1 via an Ag-C∧C∧C Intermediate

rings, a modification that can enhance solubility and enforce a more octahedral geometry about the metal center. In order to more closely mimic the geometry of the PCP and POCOP Ir complexes, we have chosen to employ the C∧C∧C ligand motif. The addition of the CH2 spacer to the ligand is anticipated to produce Ccarbene-Ir-Ccarbene bond angles similar to the P-Ir-P bond angles in the PCP and POCOP cases. In contrast to the five-membered chelate products produced by the phosphine-containing pincer ligands, this ligand modification will result in the formation of six-membered chelates. Until now, attempts to synthesize C∧C∧C-type Ir complexes have resulted in mono- or dinuclear nonpincer products, often as complex mixtures.11a,13 We now report the synthesis, structure, and reactivity of the first Ir(C∧C∧C) pincer-type complexes.

’ RESULTS The ligand precursor, [1,3-(CH2ImMes)2C6H3Br]Br2 (ImMes = 1-mesitylimidazolium), was reacted with Ag2O in CH2Cl2, yielding a dinuclear Ag-C∧C∧C complex. This light-sensitive product is formulated as depicted in Scheme 1, based on the symmetrical 1H NMR spectrum and literature precedent.14 As shown in Scheme 1, complex 1 (C∧C∧CMes)Ir(X)2 ((C∧C∧CMes = κ3-1,3-(CH2NHCMes)2C6H3, NHCMes = Nmesitylimidazol-2-ylidene) (X = 0.87 Br, 1.13 Cl)), was obtained by reaction of the silver complex with IrCl(C2H4)4, an Ir(I) starting material generated in situ.15 This reaction yields a mixture of halomers, Br:Br, Cl:Cl, and Br:Cl, which complicates characterization. Discussion of 1 hereafter is specific to the dichloride complex (C∧C∧CMes)IrCl2 (1-Cl), which was prepared separately (see Discussion). X-ray quality crystals of 1-Cl were obtained by vapor diffusion of pentane into a saturated solution of CH2Cl2. The structure is a 1430

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Figure 4. Partial (methylene region) 1H NMR spectra (500 MHz, THF-d8) of complex 1-Cl at various temperatures.

Scheme 2. Synthesis of Complex 2 via Reduction of 1 under H2

Figure 5. Partial (hydride region) 1H NMR spectrum of 2-d3 (500 MHz, THF-d8).

five-coordinate Ir(III) dihalide pincer complex (Figure 3). Selected bond lengths and angles are listed in Figure 3. The room-temperature 1H NMR spectrum of 1-Cl exhibits a broad resonance at 5.33 ppm attributable to the methylene groups. The spectrum is temperature dependent, with decoalescence of the signal observed near 275 K. Below 245 K an AB pattern is observed. The NMR spectra for 1-Cl at various temperatures are shown in Figure 4. Reduction of complex 1 with Na/Hg under 1 atm H2 results in the quantitative formation (by NMR spectroscopy) of (C∧C∧CMes)IrH4 (2), which is colorless and stable in solution for days in an inert atmosphere (Scheme 2). In addition to other appropriate ligand resonances, the 1H NMR spectrum of 2 exhibits a single hydride resonance at -9.05 ppm (4H vs ligand resonances) and one sharp singlet resonance at 5.47 ppm (4H) for the methylene groups of the ligand. The spin-lattice relaxation time (T1) of the hydride resonance in 2 was measured as a function of temperature using a standard inversion-recovery sequence. The T1(min) for the hydride resonance in 2 was found to be 75 ms at 235 K in THF-d8 (500 MHz). Under 1 atm of H2, 2 is stable indefinitely in THF, benzene, toluene, cyclohexane, and methylcyclohexane. In benzene-d6 at room temperature, removal of the H2 atmosphere leads to H/D exchange between 2 and the solvent, forming (C∧C∧CMes)IrD4 (2-d4) and C6D5H. Similar exchange reactions are observed in toluene-d8, with both aryl and benzylic positions involved, although exchange is not observed in the ortho position of toluene. After the initial H/D exchange equilibrium has been reached, the extent of H atom incorporation into the deuterated arene solvents can be increased by subsequent cycles of H2 pressurization. In contrast, solutions of 2 in deuterated alkane solvents do not show H/D exchange. In alkane solutions, slow decomposition occurs over several days, accompanied by formation of an insoluble precipitate. A value of the one-bond H-D coupling (1JHD) may be estimated from the observed H-D coupling (Jobs) in a partially deuterated sample. In order to observe H-D coupling arising only from the [Ir]HD3 isotopomer, a heavily deuterated sample of 2 (97.5% D based on integration) was prepared, resulting in the hydride signal in the 1H NMR spectrum shown in Figure 5, which shows Jobs = 3.75 Hz.

In contrast to the high thermal stability of complex 1, even mild heating of 2 (318 K) results in accelerated decomposition in alkane solvents and formation of an insoluble orange precipitate in THF, benzene, and toluene. Prolonged exposure of solid or solution samples of 2 to vacuum did not form the dihydride (C∧C∧CMes)IrH2. Upon redissolution of the dry solid in THF, the 1H NMR spectrum exhibits only signals attributable to 2. A derivative of complex 2, (C∧C∧CMes)Ir(H)2(PMe3) (3), can be readily synthesized by the addition of PMe3 to a THF solution of 2 (Scheme 3). Removal of excess phosphine and liberated H2 under reduced pressure yields complex 3 quantitatively (by NMR spectroscopy). X-ray quality crystals of 3 were obtained by slow cooling of a THF/pentane solution (1:1000). The structure is a six-coordinate cis-dihydride complex (Figure 6). Selected bond angles and lengths are listed with Figure 6. In addition to the expected C∧C∧C ligand and bound phosphine resonances, the 1H NMR spectrum of 3 exhibits two resonances in the hydride region, each a doublet of doublets (JHH = 3.6 Hz). The downfield hydride signal (JHP =136 Hz) corresponds to Ha, the hydride trans to the phosphine ligand. The upfield signal has a much smaller JHP coupling of 20 Hz and corresponds to Hb, the hydride ligand cis to the phosphine.16 The 31 1 P{ H} NMR spectrum exhibits one signal at -55.1 ppm, which is coupled to both hydride ligands, as observed by 1H-31P correlation experiments. Treatment of 2 with pyridine gives the dihydride complex (C∧C∧CMes)Ir(H)2(py) (4). Complex 4 exhibits only one hydride resonance at -8.5 ppm. This spectrum suggests a structure of higher symmetry, such as one with trans hydride ligands. In both 3 and 4, the bridging methylene groups exhibit sharp AB doublets in the 1H NMR spectra.

’ COMPUTATIONAL STUDIES Solution DFT (PBE0) calculations, using THF as the solvent medium, were employed to determine structural preferences of complex 2 and for Ir(C∧C∧CMes)H2. For comparison, we also calculated the known PCP analogues incorporating a [κ3-1,3-(CH2PtBu2)2C6H3] ligand. In every case, three isomers were located, each exhibiting different arrangements of the metal-bonded hydrogen atoms. The principal structural features and calculated ΔH/ΔG values are summarized in Figures 7 and 8, and further details are provided with the Supporting Information. The C∧C∧C and PCP complexes with four H atoms in the metal coordination sphere have similar ΔG values for the 1431

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Scheme 3. Synthesis of Dihydride Complexes 3 and 4

Figure 7. Calculated energies and schematic views of equatorial planes (oriented at 90° with respect to the pincer ligand plane) of complex 2, (C∧C∧CMes)IrH2, and the corresponding PCP complexes. and (PCP)IrH4 (ΔH/ΔG = 14.9/8.3 kcal/mol). In qualitative agreement with the experimental observations of this work, carbene complex 2 appears to be more robust with respect to H2 loss than the corresponding PCP compound.

Figure 6. ORTEP of 3, showing 50% probability ellipsoids. Hydrogen atoms, except hydride ligands, are omitted for clarity. Selected bond angles (deg) and lengths (Å): C(1)-Ir-C(12) 87.7, C(1)-Ir-C(9) 91.5, C(1)-Ir-P(1) 95.8, C(1)-Ir-Ha 88.3, C(1)-Ir-Hb 175.4, Hb-Ir-Ha 93.5, Hb-Ir-C(12) 88.3, Hb-Ir-C(9) 92.8, Hb-IrP(1) 82.0, Ha-Ir-C(9) 82.3, Ha-Ir-C(12) 82.6, Ha-Ir-P(1) 174.4, P(1)-Ir-C(12) 93.7, P(1)-Ir-C(9) 101.1, C(12)-Ir-C(9) 165.0, Ir-C(1) 2.119, Ir-C(9) 2.022, Ir-C(12) 2.000, Ir-P(1) 2.302, Ir-Ha 1.581, Ir-Hb 1.456. Ir(V) tetrahydride and Ir(III) dihydride/dihydrogen tautomers; however, Ir(V) is slightly more stabilized by the phosphine pincer. Similarly, the isomers of (C∧C∧CMes)IrH2 and (PCP)IrH2 reside on a flat potential energy surface; only the trans-(PCP)IrH2 isomer is distinctly unfavorable. The lowest free energy structure of (C∧C∧CMes)IrH2 is an Ir(I) dihydrogen complex with the H-H distance of 1.01 Å. The analogous (PCP)IrH2 complex possesses a stretched dihydrogen ligand with an H-H distance of 1.39 Å. For the latter, neutron diffraction or NMR (JHD, T1(min)) parameters are not available. However, the related POCOP = [κ3-1,3-(OPtBu2)2C6H3] complex, (POCOP)IrH2, is believed to exist as a mixture of Ir(III) dihydride and Ir(I) dihydrogen structures in solution.17 Our calculations afforded values of reaction enthalpies and free energies for H2 elimination from 2 (ΔH/ΔG = 19.8/12.9 kcal/mol)

’ DISCUSSION In order to circumvent the synthetic problems posed by direct complexation of the ligand precursor to Ir, a silver transmetalation route was pursued. Deprotonation of azolium salts using silver oxide to form Ag-NHC complexes is well-documented in the literature.18 Initial attempts at transmetalation using standard iridium reagents such as [Ir(μ-Cl)(coe)2]2 or [Ir(μ-Cl)(cod)]2 were unsuccessful, prompting a search for more reactive species. We find that reaction of the silver complex with IrCl(C2H4)4, generated in situ,15 gives a satisfactory yield of the pincer complex 1, with a mixture of chloride and bromide ligands (Scheme 1). An X-ray diffraction study of this sample indicated a composition of Br = 0.87 and Cl = 1.13. Subsequently, samples with only chloride ligands (1-Cl) were prepared by reaction of a THF solution of 2 with CCl4. Our discussion of the structure of the dihalide complex is confined to samples of 1-Cl. The square-pyramidal geometry of 1-Cl is structurally very similar to the majority of known (PCP)IrL2 and (POCOP)IrL2 complexes; however there are no known five-coordinate (CCC)Ir pincer complexes available for direct comparison. For most five-coordinate PCP and POCOP structures, the open site is located trans to a nonpincer ligand, with the meridional pincerligand plane defining the base of the pyramid.4e,19 In complex 1Cl we find the open site located trans to the aryl moiety of the 1432

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Figure 8. Views of the calculated complexes, trans-(C∧C∧CMes)IrH2(H2) and (C∧C∧CMes)IrH4. Selected bond angles (deg) and lengths (Å): (left) C1-Ir-C3 175.3, C1-Ir-C2 87.7, H1-Ir-C2 86.1, H1-Ir-C1 88.2, H1-Ir-H4 172.2, H1-Ir-H2 77.6, H2-Ir-H3 32.7, Ir-C1 2.037, Ir-C2 2.075, Ir-H1 1.667, Ir-H2 1.689, H2-H3 0.950, H1-H2 2.103; (right) C1-Ir-C3 175.2, C1-Ir-C2 87.6, H1-Ir-C2 80.1, H1-Ir-C1 90.5, H1-Ir-H4 160.3, H1-Ir-H2 70.5, H2-Ir-H3 58.9, Ir-C1 2.038, Ir-C2 2.124, Ir-H1 1.647, Ir-H2 1.608, H2-H3 1.581, H1-H2 1.880.

Figure 10. Potential hydride ligand geometries for 2.

Figure 9. Interconversion of conformations of 1-Cl in solution. Bottom sketch is a simplified projection along the Ir-phenyl bond.

pincer ligand, and the base of the pyramid comprised of the carbenic carbon and chlorine atoms. At room temperature, the broad resonance due to the methylene atoms in 1-Cl suggests isomerization (NMR time scale) of the puckered methylene bridging groups between conformers (Figure 9). A line-shape analysis of the NMR spectra at various temperatures allows determination of the thermodynamic parameters of this process. The observed values of ΔH‡ = 12.9 ( 1 kcal mol-1 and ΔS‡ = -2 ( 5 cal mol-1 K-1 are similar to the previously reported values for Pd pincer-type complexes with similar ligands.9a,20 The single resonance for the methylene groups in the roomtemperature 1H NMR spectrum of hydride complex 2 indicates a fast interconversion of the methylene protons, while the AB doublet pattern for the methylene groups in the spectra for 3 and 4 reflect static complexes. These observations indicate that the coordination of the PMe3 and pyridine ligands, respectively, prevent room-temperature interconversion in complexes 3 and 4. Complex 2 exhibits a single hydride resonance at all accessible temperatures, although no plausible static arrangement of hydride ligands could have such high symmetry. Very facile H atom permutation is well-precedented in polyhydride complexes, supporting a rapid exchange between hydridic sites in 2. In order to determine the structure of complex 2, we must consider both the experimentally determined T1(min) and JHD. Following methods outlined by Halpern and co-workers, T1(min) data can be used to detect the presence of an H2 ligand in a polyhydride complex. This analysis may be complicated by

relaxation contributions from the metal center and other ligand atoms to the overall relaxation rate, Robs (R = 1/T1).21 In the case of 2, the contribution from iridium (RIrH) is negligible. The RIrH contribution of 0.008 s-1 corresponds to only 0.06% of Robs. Similarly, using the crystal structure of 1-Cl as a guide in this system, it is possible to rule out significant contributions of the NHC-mesityl groups to the overall relaxation rate (the calculated contribution is comparable to that of Ir). Complex 2 has four structural possibilities: bis(dihydrogen), cis-dihydrogen/dihydride, trans-dihydrogen/dihydride, and tetrahydride (Figure 10). For a simple two-hydrogen-atom case, relating T1(min) of the hydride resonance to rHH is a relatively straightforward way of distinguishing between dihydride and dihydrogen ligand structures. The relationship between structure and relaxation is complicated for a dynamic polyhydride case because the T1(min) value observed reflects an averaging of the relaxation for each hydridic site. In complex 2, the T1(min) value of 75 ms makes a tetrahydride structure unlikely (it falls within the accepted range for single η2-dihydrogen ligands), but does not distinguish between the other possible structures. The four structures have been fit to an appropriate T1 model using theoretical H-H distances. While the data are most consistent with a dihydrogen/dihydride structure, definitive assignment is not possible using relaxation data alone. It is possible to infer the presence of a dihydrogen ligand in a polyhydride complex by estimating a value for the one-bond HD coupling (1JHD) from Jobs in partially deuterated samples. It is necessary to assume a statistical distribution of deuterium in all hydridic sites and small two-bond H-D couplings (2JHD).22,23 The assumption that typical 2JHD are small ((1 Hz) is generally accepted; however larger couplings have been noted. For example, in [IrH3Cp(PMe3)]BF4, where the H-H distance is 1.69 Å, 2 JHD = 3.9 Hz.24 DFT calculations on 2 suggest that the most stable structure has trans-hydride ligands, which may have a large 1433

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Organometallics trans-2JHH coupling. Coupling values of 12.5, 14.1, and 18.2 Hz have been reported for trans-dihydrides of iridium,25 rhenium,26 and iron.27 The corresponding deuterated isotopomers are expected to show 2JHD = 1.9, 2.2, and 2.8 Hz, respectively. A record large 2JHD = 7.0 Hz coupling was recently reported for trans-PdHD(IPr)(PCy3) (IPr = N,N-bis(2,6-diisopropyl)phenylimidazol-2-ylidene).28 In our case, for Jobs = 3.75 Hz (Figure 5) the use of the typical 2JHD = ( 1 Hz results in a 1JHD of 22.5 ( 5 Hz for a dihydrogen/dihydride structure. This value of 1JHD gives rHH = 1.05 ( 0.09 Å following the relation developed by Heinekey and co-workers,29 in agreement with the DFTcalculated rHH = 0.95 Å in trans-(C∧C∧C)IrH2(H2). Both the T1(min) and JHD data support our formulation of 2 as a dihydrogen/dihydride.30 However, this result should be regarded with some caution. Furthermore, the rapid H atom permutation exhibited by complex 2 does not allow us to determine whether solutions of 2 contain a single species or a mixture of Ir(III)/ Ir(V) tautomers. It is not possible to definitively assign a cis or trans geometry based on the experimental data; however the calculations strongly suggest that trans-(C∧C∧C)IrH2(H2) is more stable than the isomer with cis hydrides (ΔG = 3.0, ΔH = 3.4 kcal/mol). It is interesting to compare carbene complex 2 and the phosphine complex (PCP)IrH4 (PCP = [κ3-1,3-(CH2PtBu2)2C6H3]). The latter has a relatively more stable Ir(V) tetrahydride vs Ir(III) dihydride/dihydrogen tautomer according to DFT calculations: ΔH = -0.4 kcal/mol (PCP) vs þ0.7 kcal/mol (C∧C∧C). In agreement with the DFT data, the H-D coupling Jobs = 3.55 Hz in (PCP)IrHD3 is slightly smaller than Jobs = 3.75 Hz in 2-d3.30 Also, the T1(min) = 130 ms relaxation time in (PCP)IrH4 is longer than T1(min) = 75 ms in 2 (both values at 500 MHz). These observations are consistent with either a longer H-H distance or a greater fraction of Ir(V) in solutions of (PCP)IrH4. In this connection, it is also interesting to note that the calculated dihydrogen complex (PCP)IrH2 has a more stretched (activated) H2 ligand compared to (C∧C∧CMes)IrH2: 1.39 vs 1.01 Å, respectively. The enhanced nonclassical character of (C∧C∧CMes)IrH2 correlates with the lack of C-H activation in alkane solutions of 2. Perhaps the largest contrast between 2 and its (PCP)IrH4 analogues is the lability of bound hydrogen and formation of a dihydride species. Both (PCP)IrH4 and (POCOP)IrH4 cleanly form their corresponding dihydride complexes with heating at 403 K in vacuo; the resulting species are stable and unreactive at room temperature.19a,31 The calculated ΔG values for H2 dissociation from (PCP)IrH4 or 2 are 8.3 and 12.9 kcal mol-1, respectively. As observed above, 2 decomposes with mild heating, and extended evacuation does not result in an observable dihydride complex upon redissolving the solid. It is reasonable to assume that H2 is a labile ligand in solution, as demonstrated by the rapid H/D exchange between 2 and deuterated aromatic solvents. We hypothesize that these reactions may proceed by dissociation of H2 to form an unstable dihydride species, (C∧C∧CMes)IrH2, which decomposes at room temperature in the absence of a suitable ligand (arenes or THF).

’ CONCLUSION An Ir(C∧C∧C) pincer complex (1) was prepared by reaction of the brominated bis-azolium salt with Ag2O and subsequent transmetalation from silver to Ir using IrCl(C2H4)4. Reduction of 1 under H2 yields the dihydrogen/dihydride complex 2. The

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structural assignment of 2 is based on T1(min), JHD analysis, and DFT calculations, although the data do not rule out the possibility of 2 existing as a mixture of Ir(III) and Ir(V) tautomers in solution. The DFT and NMR data (JHD and T1(min)) for 2 are consistent with either a shorter H-H distance or a smaller fraction of an Ir(V) tautomer in solutions of this compound, compared to the phosphine analogue, (PCP)IrH4 (PCP = [κ3-1,3-(CH2PtBu2)2C6H3]). Complex 2 demonstrates H-D exchange with deuterated aromatic solvents. In alkane solution, slow decomposition was observed, with no isotope exchange. Reaction of 2 with ligands such as PMe3 and pyridine yields the six-coordinate dihydride complexes 3 and 4, respectively. Complexes 1-4 are the first examples of iridium complexes with CCC pincer ligands containing methylene bridging atoms.

’ EXPERIMENTAL SECTION General Methods. All manipulations were carried out under an atmosphere of argon in a glovebox (Vacuum Atmospheres) or using Schlenk techniques. THF-d8 was vacuum transferred from NaK/benzophenone. Cyclopentane, toluene-d8, and methylcyclohexane-d14 were vacuum transferred from NaK alloy. CD2Cl2 and C6D6 were vacuum transferred from CaH2. Solution NMR spectra were collected at room temperature (unless otherwise specified) using Bruker AV500 and DRX500 spectrometers. 1H and 13C NMR spectra were referenced to residual solvent signals, and shifts are reported in parts per million (ppm) downfield of tetramethylsilane (TMS). Relaxation rate (T1) experiments were conducted using a saturation recovery pulse sequence (180-τ-90). Temperature measurements for variable-temperature experiments were calibrated using a sample of neat methanol. Solventsuppression experiments were carried out using selective excitation with gradients.32 Elemental analyses were carried out by Columbia Analytical Services of Tucson, AZ. [Ir(μ-Cl)(coe)2]2,33 IrCl(C2H4)4,15 2-bromoR,R0 -dibromo-m-xylene,34 and 1-mesitylimidazole35 were prepared as previously described. All other materials were commercially available and were used as received, unless otherwise noted. [Ag2(C∧C∧CMes)(Br)2] and IrCl(C2H4)4 were generated in situ and were not isolated. [1,3-(CH2ImMes)2C6H3Br]Br2. This ligand precursor was prepared by a modification of the procedure reported by Braunstein and co-workers.11a A solution of 2-bromo-R,R0 -dibromo-m-xylene (2.18 g, 6.4 mmol) and 1-mesitylimidazole (2.37 g, 12.8 mmol) was refluxed in acetonitrile for 8 h. After cooling, the crude product was filtered and washed with MeCN (2  25 mL) and dried in vacuo. Yield: 3.18 g (70%). 1H NMR (CDCl3): δ 10.53 (s, 2H, im-H), 8.17 (s, 2H, im-H), 7.66 (d, 2H, 3JHH = 7.4 Hz, Ar-H), 7.40 (t, 1H, 3JHH = 7.4 Hz, Ar-H), 7.18 (s, 2H, im-H), 7.00 (s, 4H, Mes-H), 6.19 (s, 4H, CH2), 2.34 (s, 6H, p-Me), 2.08 (s, 12H, o-Me). 1,3-(CH2NHCMesAgBr)2C6H3Br. A solution of [1,3-(CH2NHCMes)2C6H3Br]Br2 (100 mg, 0.14 mmol) and Ag2O (33 mg, 0.14 mmol) was stirred in CH2Cl2 for 12 h, protected from light. The colorless productcontaining solution was filtered and carried directly into the next reaction assuming complete conversion. 1H NMR (CD2Cl2): δ 7.43 (t, 1H, 3JHH = 7.9 Hz, Ar-H), 7.31 (d, 2H, 3JHH = 1.8 Hz, im-H), 7.17 (d, 2H, 3JHH = 7.9 Hz, Ar-H), 7.02 (d, 2H, 3JHH = 1.8 Hz, im-H), 7.00 (s, 4H, Mes-H), 5.53 (br s, 4H, CH2), 2.35 (s, 6H, p-Me), 1.98 (s, 12H, o-Me). (C∧C∧CMes)Ir(X)2 (1). A solution of IrCl(C2H4)4 was prepared by bubbling C2H4 through a CH2Cl2 solution of [Ir(μ-Cl)(coe)2]2 (63 mg, 0.07 mmol) at 273 K until colorless. A CH2Cl2 solution of 1,3-(CH2NHCMesAgBr)2C6H3Br (130 mg, 0.14 mmol) was added by syringe, and the reaction was allowed to warm to 298 K and stirred for 12 h. The resulting dark brown solution was filtered through a plug of acidic alumina, and the solvent removed in vacuo to yield a bright purple solid. 1434

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Organometallics Purple, needle-like crystals were obtained by vapor diffusion of pentane into a saturated CH2Cl2 solution. Yield: 53 mg (49% based on IrC32H37N4Cl3.13Br0.87, fw = 945.26). 1H NMR (CD2Cl2): δ 7.19 (d, 2H, 3JHH = 1.8 Hz, im-H), 6.95 (d, 2H, 3JHH = 1.8 Hz, im-H), 6.92 (s, 4H, Mes-H), 6.78 (d, 2H, 3JHH = 6.7 Hz, Ar-H), 6.70 (t, 1H, 3JHH = 6.7 Hz, Ar-H), 5.33 (br s, 4H, CH2), 2.39 (s, 6H, p-Me), 1.88 (br s, 12H, o-Me). 13 C{1H} NMR (CD2Cl2): δ 172.2 (Ir-Ccarbene), 142.0 (C arom), 138.2 (C arom), 135.9 (C arom), 135.5 (C arom), 128.8 (CH mes), 125.8 (CH arom), 123.3 (CH arom), 122.7 (CH im), 120.6 (CH im), 56.0 (CH2), 20.8 (p-CH3), 19.2 (o-CH3). (C∧C∧CMes)IrH4 (2). A solution of 1 (25 mg, 0.02 mmol) in 10 mL of THF over 0.2 mL of NaHg amalgam (0.5%) was stirred under H2(g) in a Kontes bomb for 4 h at 298 K. The solution was removed from the NaHg by syringe, and volatiles were removed in vacuo to yield a colorless residue. Attempts at recrystallization were unsuccessful; redissolution in THF-d8 indicated only one product. 1H NMR (THF-d8): δ 7.18 (d, 2H, 3 JHH = 1.8 Hz, im-H), 6.96 (d, 2H, 3JHH = 7.3 Hz, Ar-H), 6.83 (s, 4H, Mes-H), 6.75 (t, 1H, 3JHH = 7.3 Hz, Ar-H), 6.74 (d, 2H, 3JHH = 1.8 Hz, im-H), 4.54 (s, 4H, CH2), 2.31 (s, 6H, p-Me), 1.69 (s, 12H, o-Me), 9.04 (s, 4H). 13C{1H} NMR (THF-d8): δ 165.9 (Ir-Ccarbene), 145.5 (C arom), 140.5 (C arom), 138.2 (C arom), 137.1 (C arom), 135.9 (C arom), 128.1 (CH mes), 122.9 (CH arom), 120.2 (CH arom), 119.1 (CH im), 118.6 (CH im), 62.4 (CH2), 20.2 (p-CH3), 17.8 (o-CH3). (C∧C∧CMes)IrHD3 (2-d3). A solution of 2 (5 mg, 0.007 mmol) in 1 mL of C6D6 was allowed to react with solvent under reduced pressure in a J. Young NMR tube for 3 days until only the resonance arising from [Ir]HD3 was visible in the hydride region of the 1H NMR spectrum. (C∧C∧CMes)Ir(Cl)2 (1-Cl). A solution of 2 (25 mg, 0.04 mmol) in 5 mL of THF was reacted with excess CCl4 (0.5 mL) and then stirred for 12 h at 25 °C. The resulting orange solution was filtered through a plug of acidic alumina, and the solvent removed in vacuo to yield a bright purple solid. The product was crystallized by vapor diffusion of pentane into a CH2Cl2 solution. Yield = 7 mg (24%). 1H NMR (CD2Cl2): δ 7.19 (d, 2H, 3JHH = 1.8 Hz, im-H), 6.95 (d, 2H, 3JHH = 1.8 Hz, im-H), 6.92 (s, 4H, Mes-H), 6.78 (d, 2H, 3JHH = 6.7 Hz, Ar-H), 6.70 (t, 1H, 3JHH = 6.7 Hz, Ar-H), 5.33 (br s, 4H, CH2), 2.39 (s, 6H, p-Me), 1.88 (br s, 12H, oMe). 13C{1H} NMR (CD2Cl2): δ 172.2 (Ir-Ccarbene), 142.0 (C arom), 138.2 (C arom), 135.9 (C arom), 135.5 (C arom), 128.8 (CH mes), 125.8 (CH arom), 123.3 (CH arom), 122.7 (CH im), 120.6 (CH im), 56.0 (CH2), 20.8 (p-CH3), 19.2 (o-CH3). Anal. Calcd for IrC32H33N4Cl2: C, 52.1; H, 4.5; N, 7.6. Found: C, 51.5; H, 5.2; N, 7.3. (C∧C∧CMes)Ir(H)2(PMe3) (3). Excess PMe3 (0.2 mL, 1.9 mmol) was added via syringe to a solution of 2 (20 mg, 0.03 mmol) in 5 mL of THF under H2(g). The solution was frozen, and the headspace evacuated. Removal of solvent and unreacted PMe3 in vacuo yielded 3 as a white solid. Redissolution in THF-d8 indicated only one product by NMR. 1H NMR (THF-d8): δ 7.15 (d, 2H, 3JHH = 1.9 Hz, im-H), 6.80 (br s, 2H, Mes-H), 6.78 (d, 2H, 3JHH = 7.3 Hz, Ar-H), 6.74 (d, 2H, 3JHH = 1.9 Hz, im-H), 6.72 (br s, 2H, Mes-H), 6.62 (t, 1H, 3JHH = 7.3 Hz, Ar-H), 5.01 (d, 2H, 2JHH = 14.1 Hz, CH2), 4.79 (d, 2H, 2JHH = 14.1 Hz, CH2), 2.32 (s, 6H, p-Me), 1.92 (s, 6H, o-Me), 1.72 (s, 6H, o-Me), 0.87 (d, 9H, 2JHP = 7.1 Hz, Me3), -11.23 (d, 1H, 2JHH = 3.6 Hz, 2JHp = 136 Hz, Ha), -13.37 (d, 1H, 2JHH = 3.6 Hz, 2JHp = 20 Hz, Hb). 13C{1H} NMR (THF-d8): δ 168.4 (Ir-Ccarbene), 140.5 (C arom), 139.1 (C arom), 137.6 (C arom), 136.2 (C arom), 134.4 (C arom), 128.5 (CH mes), 127.4 (CH mes), 122.2 (CH arom), 119.9 (CH im), 119.5 (CH im), 118.3 (CH arom), 61.5 (CH2), 23.2 (C PMe3), 20.4 (C o-CH3) 20.2 (p-CH3), 18.6 (oCH3). 31P{1H} NMR (THF-d8): δ -55.1 ppm. Anal. Calcd for IrC35H44N4P: C, 56.5; H, 6.0; N, 7.5. Found: C, 54.8; H, 6.5; N, 6.5. (C∧C∧CMes)Ir(H)2(py) (4). Excess pyridine (0.3 mL, 3.7 mmol) was added via syringe to a solution of 2 (20 mg, 0.03 mmol) in 5 mL of THF under H2(g). The solution was frozen, and the headspace evacuated. Removal of solvent and unreacted pyridine in vacuo yielded 4 as a yellow solid. Redissolution in THF-d8 indicated only one product by NMR. 1H

ARTICLE

Table 1. Crystal Data and Structure Refinement for 1-Cl and 3 color, shape

purple, plates

colorless, prisms

empirical formula

C34H37N4Cl6Ir

C35H44N4PIr

fw

906.58

743.91

radiation (Å)

Mo, 0.71073

Mo, 0.71073

temperature (K)

110(2)

100(2)

cryst syst

monoclinic

monoclinic

space group

P21/n

P21/c

a (Å) b (Å)

7.9882(2) 21.6841(6)

11.1573(3) 10.3737(3)

c (Å)

20.0752(5)

28.4376(7)

R (deg)

90

90

β (deg)

90.1490(10)

97.465(2)

γ (deg)

90

90

volume (Å3)

3477.35(16)

3263.54(15)

Z

4

4

densitycalc (g cm-3) μ(Mo) (mm -1)

1.732 4.332

1.514 4.170

cryst size (mm3)

0.30  0.18  0.03

0.10  0.06  0.05

θ range for data coll (deg)

1.88 < θ < 28.53

1.84 < θ < 28.37

total, unique no. of reflns

118 041, 8686

53 087, 8133

Rint

0.0371

0.1050

no. of params, restraints

413, 0

387, 0

R, wR2 for all data

0.0438, 0.0859

0.0741, 0.0677

R, wR2 for I > 2σ(I) GOF on F2

0.0363, 0.0796 1.110

0.0389, 0.0596 1.010

resid density (e Å-3)

2.969, -1.256

1.067, -1.110

NMR (THF-d8): δ 8.26 (d, 2H, 3JHH = 5.5 Hz, py-H), 7.18 (d, 2H, 3JHH = 1.9 Hz, im-H), 7.11 (t, 1H, 3JHH = 7.3 Hz, py-H), 6.76 (br s, 2H, MesH), 6.69 (d, 2H, 3JHH = 7.3 Hz, Ar-H), 6.49 (br s, 2H, Mes-H), 6.41 (t, 1H, 3JHH = 7.3 Hz, Ar-H), 6.15 (d, 2H, 3JHH = 1.9 Hz, im-H), 6.08 (m, 2H, 3JHH = 5.5 Hz, 7.3 Hz, py-H), 5.14 (d, 2H, 2JHH = 11.0 Hz, CH2), 4.79 (d, 2H, 2JHH = 11.0 Hz, CH2), 2.18 (br s, 6H, p-Me), 2.06 (s, 6H, oMe), 1.23 (br s, 6H, o-Me), -8.77 (s, 2H). 13C{1H} NMR (THF-d8): δ 173.2 (Ir-Ccarbene), 161.4 (C py), 141.5 (C arom), 137.9 (C arom), 136.9 (C arom), 136.3 (C arom), 134.9 (C arom), 129.6 (C py), 128.4 (CH arom), 127.9 (CH mes), 123.1 (C py), 121.4 (CH mes), 119.1 (CH im), 118.8 (CH im), 116.2 (CH arom), 62.1 (CH2), 20.0 (C pCH3) 19.6 (o-CH3), 17.1 (o-CH3). Anal. Calcd for IrC37H40N5: C, 59.5; H, 5.4; N, 9.4. Found: C, 59.4; H, 5.9; N, 9.0. Computational Details. All calculations were carried out in Gaussian 09 (A.02) using the PBE1PBE (also known as PBE0) functional, tight optimizations, and the ultrafine integration grid (a pruned (99,590) grid).36 The basis sets included Def2-QZVPP (with the corresponding ECP)37 for Ir, 6-31þG(3pd) for metal-bonded hydrogen atoms, 6-311þG(d) for P and Cl, and 6-31þG(d,p) for all other atoms. The default PCM method was used for all calculations (solvent = tetrahydrofuran) with the UFF radii scaled by 1.1 (explicit hydrogens). The optimized geometries were verified to have all real harmonic frequencies by frequency calculations, which also provided the enthalpies and free energies reported here, all at 298.15 K. The free energies were calculated at P = 302 atm.38 X-ray Crystallography, General Methods. X-ray quality crystals of 1Cl were obtained by vapor diffusion of pentane into a saturated solution of CH2Cl2. The sample examined crystallized with two molecules of CH2Cl2 in the lattice. X-ray quality crystals of 3 were obtained by slow cooling of a THF/pentane solution (1:1000). Structure determinations were performed on a Bruker APEX II single-crystal X-ray diffractometer, 1435

dx.doi.org/10.1021/om101024x |Organometallics 2011, 30, 1429–1437

Organometallics using Mo radiation. Crystal, measurement, and refinement data are given in Table 1. The data were integrated and scaled using SAINT and SADABS within the APEX2 software package.39 Solution by direct methods (SHELXS, SIR9740) produced a complete heavy atom phasing model consistent with the proposed structures. The structures were completed by difference Fourier synthesis with SHELXL97.41,42 Scattering factors are from Waasmair and Kirfel.43 Hydrogen atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms with C-H distances in the range 0.95-1.00 Å. Isotropic thermal parameters (Ueq) were fixed such that they were 1.2Ueq of their parent atom Ueq for C-H and 1.5Ueq of their parent atom Ueq in the case of methyl groups. All non-hydrogen atoms were refined anisotropically by full-matrix least-squares. The metal-bound H atoms in 3 were located from the Fourier map and refined with isotropic thermal ellipsoids.

’ ASSOCIATED CONTENT

bS

Supporting Information. Computational details and full ref 36a; NMR spectra of (C∧C∧CMes)Ir(H)2(PMe3) (3) and (C∧C∧CMes)Ir(H)2(py) (4); complete crystallographic data in CIF format for (C∧C∧CMes)IrCl2 (1-Cl) and (C∧C∧CMes)Ir(H)2(PMe3) (3). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was made possible by the facilities of the Shared Hierarchical Academic Research Computing Network (SHARCNET: www.sharcnet.ca) and by the National Science Foundation as part of the Center for Enabling New Technologies through Catalysis (CENTC), CHE-0650456. ’ REFERENCES (1) The Chemistry of Pincer Compounds; Morales-Morales, D.; Jensen, C. M., Eds.; Elsevier: Amsterdam, The Netherlands, 2007. (2) (a) Albretch, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750. (b) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759. (3) Morales-Morales, D. Mini-Rev. Org. Chem. 2008, 5. (4) (a) Liu, F.; Pak, E. B.; Singh, b; Jensen, C. M.; Goldman, A. S. J. Am. Chem. Soc. 1999, 121, 4086. (b) Kanzelberger, M.; Singh, B.; Czerw, M.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2000, 122, 11017. (c) Krogh-Jespersen, K.; Czerw, M.; Summa, N.; Renkema, K. B.; Achord, P. D.; Goldman, A. S. J. Am. Chem. Soc. 2002, 124, 11404. (d) Renkema, K. B.; Kissin, Y. V.; Goldman, A. S. J. Am. Chem. Soc. 2003, 125, 7770. (e) G€ottker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804. (f) G€ottker-Schnetmann, I.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 9330. (g) Wang, Z.; Tonks, I.; Belli, J.; Jensen, C. J. Organomet. Chem. 2009, 694, 2854. (5) (a) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 12048. (b) Dietrich, B. L.; Goldberg, K. I.; Heinekey, D. M.; Autrey, T.; Linehan, J. C. Inorg. Chem. 2008, 47, 8583. (6) (a) Crabtree, R. H. J. Organomet. Chem. 2005, 690, 5451. (b) Hu, X.; Meyer, K. J. Organomet. Chem. 2005, 690, 5474. (7) (a) Klei, S. R.; Tilley, T. D.; Bergman, R. G. J. Am. Chem. Soc. 2000, 122, 1816. (b) Strout, D. L.; Zari, S.; Niu, S.; Hall, M. B. J. Am. Chem. Soc. 1996, 118, 6068. (c) Niu, S.; Hall, M. B. J. Am. Chem. Soc. 1998, 120, 6169.

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Organometallics

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